1. Field of the Invention
The present invention relates generally to circuitry for conditioning alternating current waveforms to produce amplified and rectified waveforms. In particular, the invention relates to a technique for receiving AC waveforms of a fairly large dynamic range, rectifying the waveform, and amplifying the waveform by one of a plurality of discrete gain levels in a closed loop feedback configuration for obtaining waveforms suitable for input into downstream circuitry, such as an analog-to-digital converter.
2. Description of the Related Art
A variety of applications exist for signal processing of alternating current waveforms wherein the input waveform must be rectified and amplified for application to downstream circuitry. For example, in a current sensing relay, current sensors may be applied to one or more current-carrying conductors for outputting signals which are indicative of a level of current flow. Depending upon the type of downstream processing, the signal may need to be rectified and digitized, particularly where downstream circuitry includes digital signal processing circuitry such as microprocessors, digital signal processors, and the like. In such arrangements, circuitry must not only rectify the input signal, but may need to amplify the input signal to make best use of the dynamic range of an analog-to-digital converter. The amplification becomes somewhat more complex in applications where the dynamic range of the input signal itself may vary widely.
In applications including analog-to-digital converters and input signals comprising AC waveforms of a broad dynamic range, difficulties may be encountered in the scaling of the rectified waveform to make best use of the dynamic range of the analog-to-digital converter, while avoiding excessive amplification of noise. For example, where an input signal to such circuitry is an AC waveformn, a very low amplitude may result in output data from the analog-to-digital converter which is of little utility due to a lack of sufficient amplification. On the contrary, where an input signal has a dynamic range which may change substantially during operation, a fixed amplification level may cause the analog-to-digital converter output to saturate when the amplitude of the input signal increases substantially as compared to its normal amplitude levels, or at least to the amplitude levels at which the amplification gain was appropriate.
In monitoring and control equipment, such as microprocessor-based overload relays, very substantial dynamic ranges may be encountered in input levels of AC waveforms, such as from current sensors. To perform analysis of the input signals, however, the signals must be rectified and digitized. Accommodation of the large variations in the amplitude of the input signal requires a novel approach to both the rectification and the amplification of the signal prior to application of the output to the analog-to-digital converter.
In general, analog-to-digital converters may not sample negative portions of an input signal, such devices generally operating between an input range of 0 to 5 volts. Thus, precision full wave rectifiers are typically needed to provide an absolute value function, affording proper operation of the analog-to-digital converter. Traditional full wave rectifiers have been employed for this purpose, including a pair of cascaded amplifiers to produce the absolute value function. However, such devices often produce intolerable levels of error due to the amplification of the first stage amplifiers error by the second amplifier, and addition of this amplified error to the error of the second amplifier itself. Moreover, conventional precision full wave rectifiers may offer gain, but do not offer adjustable gain. Such adjustability in gain levels would be highly desirable to increase the dynamic range of the system, but such adjustability is difficult to synthesize in a non-cascaded amplifier approach.
There is a need, therefore, for a technique capable of rectifying and amplifying AC waveforms of varying amplitude. For practical applications, the technique should be relatively easy to implement and cost effective to manufacture. Moreover, there is a particular need for a technique which provides discrete levels of amplification based upon the level of an output waveform applied to downstream circuitry, such as an analog-to-digital converter. In circuits including a digital signal processor, a microprocessor or a similar programmable device, it would be particularly convenient to provide some degree of feedback control of the amplification level based upon detected and fed-back characteristics of the output waveform.